The disclosure of U.S. Provisional Application No. 61/527,530, filed Aug. 25, 2011, is hereby incorporated herein in its entirety by reference.
Transmissions with hydraulically operated park brakes need to consistently operate such that the timing of brake engagement and disengagement can be controlled precisely. This can be difficult, since the brakes are mechanically applied by springs, the brakes typically have multiple plates, and there is variation between springs and other component tolerances, such that the pressure needed to move the piston to the point at which the plates touch and the brake begins to engage and effect torque transmission can vary quite a bit.
Most transmissions up to now in tractors have been power shift. Continuously variable transmissions or CVTs are becoming more popular and have significantly different architectures, often using a hydrostatic pump and motor. The pressure between the pump and motor is a direct indication of the torque through the hydrostatic unit, and in turn an indication of the torque absorbed, and the effect on torque transmission, by the park brake. In some designs, the hydraulic pump and motor can be in separate housings, connected with tubes or hoses, but they may be in the same housing (a hydrostatic unit or HSU), which typically increases efficiency.
Typically, known electronically controlled transmissions with clutches use a calibration routine. A searching method is used to determine the current needed to provide the pressure to just start to engage the clutch. This point is usually determined by when the engine speed reduces or engine torque changes, indicating that the clutch is beginning to engage. However, a shortcoming with this approach is that engine speed can vary as a result of other reasons, such as when other loads on the engine change, such as hydraulic or PTO loads, and various drags on the engine vary and such. Also, tractor engines have a governor that is quite complex, and may change engine speeds due to complex algorithms to manage emissions, efficiency and other factors. As a result, how much the engine speed dips cannot always be a direct indication of the level of torque on a park brake, which can result in different calibration values. Park brake calibration done using engine torque would have similar shortcomings. A torque sensor could also be used for calibration, but such torque sensors are expensive and add complexity. Optionally, adding another torque sensor might be one costly option. Also, engine torque could be estimated from fuel consumption, but this would be a slower signal, and again subject to other loads on the engine.
Thus, what is sought is a manner of hydraulic park brake calibration that does not rely on engine speed, torque or separate torque sensors, and overcomes one or more of the shortcomings set forth above.